fix(#1831): resolve ESLint errors and unused-var warnings from #1829/#1830

Two hard errors in src/state-transition.cts:
- reconcileCurrentPosition: `!== null` guards on `Record<string,unknown>`
  values don't narrow the type to a primitive, causing
  @typescript-eslint/no-base-to-string to fire on String(fm.current_phase)
  and String(fm.current_phase_name). Extract to typed locals + use typeof
  narrowing so the rule sees string | number — which is the actual invariant.

Four unused-var warnings (warnings are still defects per RULESET):
- stripTemplatePlaceholders: `placeholder` was assigned value.trim() but
  never read — the value came from the loop variable itself, so removed.
- deduplicateSessionArchive: `sectionContent` was sliced but the filter
  below uses the raw hs offsets directly — variable was dead code; removed.
- state-rebuild.test.cjs: first transitionCore call in the orphan-row test
  is covered by the dedicated Leaky-Abstractions guard test below it;
  remove the no-op call rather than silently ignoring its result.
- state-rebuild.test.cjs: `before = state` in the sync regression guard
  test was never read — remove the dead assignment.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/state-transition.cts

@@ -1666,8 +1666,9 @@ function reconcileCurrentPosition(
   // by other transitions (beginPhase / completePhase) and reconstructed from
   // total-phase counts; rebuild reconciles only the `**Current Phase:**` body
   // field that frontmatter is the canonical source for.
-  if (fm.current_phase !== undefined && fm.current_phase !== null) {
+  const fmPhase = fm.current_phase;
-    const canonicalPhase = String(fm.current_phase);
+  if (typeof fmPhase === 'string' || typeof fmPhase === 'number') {
+    const canonicalPhase = String(fmPhase);
     const existing = stateExtractField(modified, 'Current Phase');
     if (existing !== null && existing !== canonicalPhase) {
       const replaced = stateReplaceField(modified, 'Current Phase', canonicalPhase);
@@ -1686,8 +1687,9 @@ function reconcileCurrentPosition(
   }
 
   // Phase name prose.
-  if (fm.current_phase_name !== undefined && fm.current_phase_name !== null) {
+  const fmPhaseName = fm.current_phase_name;
-    const canonicalName = String(fm.current_phase_name);
+  if (typeof fmPhaseName === 'string' || typeof fmPhaseName === 'number') {
+    const canonicalName = String(fmPhaseName);
     const existing = stateExtractField(modified, 'Current Phase Name');
     if (existing !== null && existing !== canonicalName) {
       const replaced = stateReplaceField(modified, 'Current Phase Name', canonicalName);
@@ -1821,7 +1823,6 @@ function stripTemplatePlaceholders(
     const fieldName = m[1];
     const value = m[2];
     if (TEMPLATE_PLACEHOLDER_VALUE.test(value)) {
-      const placeholder = value.trim();
       const cleared = `**${fieldName}:** (pending)`;
       replacements.push({ lineIdx: i, before: line, after: cleared, fieldName });
     }
@@ -1874,8 +1875,6 @@ function deduplicateSessionArchive(
   for (let i = sectionIdx + 1; i < hs.length; i++) {
     if (hs[i].level === 2) { sectionEnd = hs[i].offset; break; }
   }
-  const sectionContent = content.slice(sectionStart, sectionEnd);
-
   // Count `### Session — …` H3 sub-headings inside the section.
   const archiveHeadings = hs.filter(
     (h) => h.level === 3 && h.offset >= sectionStart && h.offset < sectionEnd && SESSION_ARCHIVE_H3.test(h.text),

tests/state-rebuild.test.cjs

@@ -367,8 +367,7 @@ describe('ADR-1817 §2: rebuild reconciles **By Phase:** table via phaseInventor
         { number: '3', name: 'Test Phase', planCount: 5, summaryCount: 4 },
       ],
     };
-    const result = transitionCore(drifted, { kind: 'rebuild' }, baseDeps);
+    // First call with no phaseInventoryProvider → no-op (covered by its own test below).
-    // Note: passing baseDeps here (no provider) → reconcile is a no-op.
     // Re-run with the provider-wired deps:
     const result2 = transitionCore(drifted, { kind: 'rebuild' }, deps);
     // Strip the audit log so we only inspect LIVE table rows (the log's
@@ -421,7 +420,6 @@ describe('ADR-1817 §2: rebuild reconciles **By Phase:** table via phaseInventor
 describe('ADR-1817 §5/§6: rebuild does not affect sync or prune (regression guard, criterion #7)', () => {
   test('sync still patches its three frontmatter fields when rebuild is also available', () => {
     const state = cleanState();
-    const before = state;
     const result = transitionCore(
       state,
       { kind: 'sync', totalPlansInPhase: 7, percent: 50 },